1. Field of the Invention
The present invention relates to a power transmitting system for a motor vehicle having a continuously variable transmission and a hydraulic control apparatus for controlling the speed ratio of the transmission.
2. Discussion of the Prior Art
An example of a hydraulic control apparatus for a continuously variable transmission for a motor vehicle is disclosed in laid-open Publication No. 61-197849. This hydraulic control apparatus uses a shift control valve including a valve spool which is biased in a direction to shift down the transmission, by a thrust corresponding to the currently required output of the vehicle engine, and in a direction to shift up the transmission, by a thrust corresponding to the speed of the input shaft of the transmission. The speed ratio of the transmission is changed according to the distance and direction of movement of the spool from the neutral position.
In the shift control valve of the above hydraulic control apparatus, the thrust corresponding to the currently required output of the engine represented by the throttle valve opening acts on the spool through a spring. The constant of this spring changes in two steps, so that the spool can be moved with a better response to a change in the input shaft speed (as represented by a pressure produced by a pitot tube) when the throttle valve opening is relatively small than when it is relatively large. This arrangement permits the speed ratio of the transmission to be rapidly changed in the shift-down direction when abrupt brake is applied to the vehicle.
While the above known hydraulic control apparatus is adapted to improve the shifting response of the transmission by changing the constant of the spring depending upon the throttle valve opening, the shift control valve to change the speed ratio of the transmission becomes excessively sensitive when the vehicle speed is relatively high and when the throttle valve opening is relatively small. Further, during a high-speed running of the vehicle, the speed ratio is reduced toward the minimum value in the shift-up direction, and the shifting response increases as the speed ratio approaches the minimum value. The above tendencies are not desirable for smooth running of the vehicle. Since the continuously variable transmission has a characteristic associated with the rotating speed of the transmission belt that the shifting response decreases and increases as the speed ratio approaches the maximum and minimum values, respectively, the hydraulic shifting response is desirably positively decreased when the vehicle is running at a low speed with the speed ratio of the transmission being controlled to a relatively low value.